Neal Walfield [interview] has committed a POSIX semaphores implementation for libpthread, closing a major feature hole which should make porting of more complicated applications possible. He also fixed several bugs and issues in libpthread itself, which were exposed by various test suites and provided a status report.

From: Neal H. Walfield
Subject: libpthread status report
Date: Tue, 17 May 2005 



I've run the semaphore and thread parts of posixtestsuite 1.5.0 (with
a bunch of patches because a number of their tests are not POSIX
compliant) against our libpthread.  We pass most tests.  Here are the
major failings.

- signal handling

  POSIX signal handling is completely broken (both kill and
  pthread_kill).  signals are delivered to the wrong threads, etc.
  This requires investigation.

  Also, pthread_kill does not correctly test if signal number is
  valid.

- cancellation

  Cancellation is not fully integrated into libc.  For example, sleep
  needs to be a cancellation point.
  
  There is a bug in pthread_once.  POSIX says: "if init_routine is a
  cancellation point and is canceled, the effect on once_control shall
  be as if pthread_once() was never called."

- pthread_atfork

  Depends on support from glibc.

- user stacks

  We allow user stacks but only if they are 2MB and a aligned on a 2MB
  boundary (this is because TSD is stored at the base of the stack).
  Programs would have a better chance of using this feature if we set
  PTHREAD_STACK_MIN to 2MB but there are still alignment issues.  If
  we set it to about 4MB, we could just find the lowest address that
  is a multiple of 2MB and use that as the base.  It would cost a bit
  of virtual address space but shouldn't be too bad.  On the other
  hand, we haven't encountered a program that relies on this feature.

- process shared synchronization

  We have no pshared support for mutexes etc.  I could come up with a
  hack here such as busy waiting on the memory but again, no programs
  have required this.

- shm_open and shm_unlink

  Unsupported, however, most applications seem to use anonymous
  semaphores.  To support them we would need to support the pshared
  attribute.

- 

  There are various limits which need to be defined by ,
  e.g. PTHREAD_KEYS_MAX.  We don't define them at all right now.  As a
  hack, we could define them in .
  
  These usually have sysconf equivalents.  I don't think this can be
  worked around in libpthread.  I think we need support from glibc.

- scheduling

  We don't support scheduling attributes, priority ceilings, etc.

The following failures were not specific to libpthread:

- clock_gettime

  A large number of tests failed because we don't have clock_gettime.
  glibc is supposed to provide this.

- RLIMIT_AS

  glibc doesn't define it.


Thanks,
Neal